1. Field of the Invention
The present invention relates to a receiving apparatus and method for a Single Carrier-Frequency Division Multiple Access (SC-FDMA) system. More particularly, the present invention relates to a receiving apparatus and method for an SC-FDMA system, whereby a multi-path channel is converted into a single path channel, channel compensation is performed in a frequency domain, and a received symbol is processed in a time domain.
2. Description of the Related Art
A technique for reducing a Peak-to-Average Power Ratio (PAPR) and ensuring orthogonality between users has recently emerged as an important issue for improving performance in uplink communication. An Orthogonal Frequency Division Multiple Access (OFDMA) scheme is widely used but has a demerit in that the PAPR is high. In general, the PAPR is problematic when one user uses multiple carriers or multiple codes as in the case of Code Division Multiple Access (CDMA) scheme. Therefore, in terms of PAPR performance, a method of using a single carrier is preferred. A Single Carrier-Frequency Division Multiple Access (SC-FDMA) scheme is a prominent single carrier scheme discussed in the 3rd Generation Partnership Project (3GPP).
FIG. 1A illustrates a Multiple Input Multiple Output (MIMO) receiver in a conventional SC-FDMA system.
The receiver of FIG. 1A includes an N-Fast Fourier Transform (FFT) processor 100, a channel estimator 102, a linear MIMO detector 104, a plurality of Inverse Discrete Fourier Transform (IDFT) units 106-1 to 106-L, and a Forward Error Correction (FEC) decoder 108.
The N-FFT processor 100 converts a Radio Frequency (RF) signal received through at least one Receive (Rx) antenna into a baseband signal and converts the baseband signal into a frequency-domain signal by performing an FFT operation.
The channel estimator 102 de-maps respective data symbol signals from subcarriers, wherein the data symbol signals are converted into frequency-domain signals by the FFT unit 100 and estimates respective channels by using pilot signals from among the output signals.
The linear MIMO detector 104 compensates for the data symbol signals de-mapped from the subcarriers in a frequency domain by using respective channel estimation values estimated by the channel estimator 102. Then, the linear MIMO detector 104 divides the resultant signals into L MIMO layers (e.g., corresponding to the number of flows transmitted from a transmitter) and outputs the signals to the respective IDFT units 106-1 to 106-L. An IDFT size of the IDFT unit 106 varies according to an amount of a resource allocated to each user. The linear MIMO detector 104 may use a Minimum Mean Squared Error (MMSE) scheme.
The IDFT units 106-1 to 106-L perform an IDFT operation on signals which are output for respective layers separated by the linear MIMO detector 104, and generate soft-out values for decoding.
The FEC decoder 108 decodes the soft-out values received from the IDFT units 106-1 to 106-L and performs error correction on decoded information bits.
As described above, when MIMO detection is performed, the linear MIMO receiver can separate the MIMO layers and can generate the soft-out value for decoding by using only a MIMO channel matrix irrespective of a Transmit (Tx) symbol vector. Therefore, in a manner similar to the OFDMA system, the SC-FDMA system also compensates for a channel for each subcarrier in the frequency domain and separates the MIMO layers, and thereafter generates the soft-out values by performing an IDFT operation. However, when the linear MIMO receiver is used in the SC-FDMA system, unlike in the OFDMA system, a Tx signal vector of the frequency domain is first separated and is then subjected to the IDFT operation to generate soft-out values for a Tx symbol vector of a time domain.
Meanwhile, the performance of the linear MIMO receiver is basically inferior to that of a non-linear MIMO receiver. To overcome this problem, a Maximum-Likelihood (ML)-based non-linear MIMO receiver providing excellent performance can be considered for use in the SC-FDMA system.
FIG. 1B illustrates a conventional ML-based nonlinear MIMO receiver.
The receiver of FIG. 1B includes an N-FFT processor 101, a channel estimator 102, a plurality of IDFT units 103-1 to 103-NR, an ML MIMO detector 105, and an FEC decoder 107. The N-FFT processor 101, the channel estimator 102, and the FEC decoder 107 are the same as the N-FFT processor 100, the channel estimator 102, and the FEC decoder 108 described in FIG. 1A, and thus detailed explanations thereof will be omitted.
The N-FFT processor 101 converts an RF signal received through at least one Rx antenna into a baseband signal and converts the baseband signal into a frequency-domain signal by performing an FFT operation.
The IDFT units 103-1 to 103-NR perform an IDFT operation on signals which have undergone the FFT operation and thus convert the signals into time-domain signals. The time-domain signals are output to the ML MIMO detector 105. An IDFT size (i.e., NIDFT) of the IDFT unit 103 varies according to an amount of a resource allocated to each user.
The channel estimator 102 de-maps respective data symbol signals from subcarriers, wherein the data symbol signals are converted into frequency-domain signals by the FFT unit 101. Then, the channel estimator 102 estimates respective channels by using pilot signals from among the output signals.
Unlike the linear MIMO detector 104 of FIG. 1A, the ML MIMO detector 105 determines an ML criterion by using a candidate Tx symbol vector and a channel matrix estimated by the channel estimator 102. The ML criterion for ML-based MIMO reception in the frequency domain of the SF-FDMA system can be expressed by Equation (1) below.
                                                                                          S                  ^                                k                            =                                                min                                      S                    k                                                  ⁢                                                                                                                        R                        k                                            -                                                                        H                          k                                                ⁢                                                  S                          k                                                                                                                          2                                                                                                        =                                                min                                                            S                      0                                        ,                    …                    ⁢                                                                                  ,                                          S                                                                        N                          DFT                                                -                        1                                                                                            ⁢                                                                                                                        R                        k                                            -                                                                        H                          k                                                ⁢                                                  {                                                                                    ∑                                                              n                                =                                0                                                                                                                              N                                  DFT                                                                -                                1                                                                                      ⁢                                                                                          s                                n                                                            ⁢                                                              exp                                ⁡                                                                  (                                                                                                            -                                      j                                                                        ⁢                                                                                                                  2                                        ⁢                                                                                                                                                                  ⁢                                        π                                        ⁢                                                                                                                                                                  ⁢                                        nk                                                                                                                    N                                        DFT                                                                                                                                              )                                                                                                                                              }                                                                                                                          2                                                                                        [                  Eqn          .                                          ⁢          1                ]            
In Equation (1), Rk denotes an Rx signal vector, Hk denotes a channel matrix, Sk denotes a candidate Tx symbol vector in the frequency domain, NDFT denotes a DFT size, sn denotes a candidate Tx symbol vector in the time domain and k denotes a subcarrier index.
The FEC decoder 107 decodes soft-out values provided from the ML MIMO detector 105 and performs error correction on the decoded information bits.
Referring to Equation (1) above, in the frequency domain of the SC-FDMA system, the ML criterion has to be determined not for a candidate Tx symbol vector but for a DFT-converted candidate Tx signal vector. Therefore, if the number of Tx streams is equal to the number of Tx antennas and if the same modulation scheme is used in which a size of a signal constellation point is |C|, a computational amount is increased by (|C|Nr)NDFT to obtain a size of a candidate signal vector. In the conventional ML-based MIMO receiver, the candidate signal vector of an OFDMA system has a size of (|C|Nr). In comparison thereto, the candidate signal vector of the SC-FDMA system has a size that increases exponentially with NDFT with respect to complexity of the conventional ML-based MIMO receiver. Disadvantageously, the ML-based MIMO receiver cannot be implemented in practice in the frequency domain of the SC-FDMA system.
In contrast, when ML-based MIMO reception is achieved in the time domain, the ML criterion for ML-based MIMO reception can be expressed by Equation (2) below.
                                                        s              ^                        n                    =                                    min                                                s                  n                                |                                  s                                      n                    -                                          d                      i                                              r                        ,                        t                                                                                                                  ⁢                                                                                                r                    n                                    -                                                            [                                                                        ∑                                                      t                            =                            1                                                                                N                            T                                                                          ⁢                                                                              ∑                                                          i                              =                              0                                                                                                                      P                                                                  1                                  ,                                  t                                                                                            -                              1                                                                                ⁢                                                                                    h                              i                                                              1                                ,                                t                                                                                      ⁢                                                          s                                                              n                                -                                                                  d                                  i                                                                      1                                    ,                                    t                                                                                                                                                        ⁢                                                                                                                  ⁢                            …                            ⁢                                                                                                                  ⁢                                                                                          ∑                                                                  t                                  =                                  1                                                                                                  N                                  T                                                                                            ⁢                                                                                                ∑                                                                      i                                    =                                    0                                                                                                                                              P                                                                                                                        N                                          R                                                                                ,                                        t                                                                                                              -                                    1                                                                                                  ⁢                                                                                                      h                                    i                                                                                                                  N                                        R                                                                            ,                                      t                                                                                                        ⁢                                                                      s                                                                          n                                      -                                                                              d                                        i                                                                                                                              N                                            R                                                                                    ,                                          t                                                                                                                                                                                                                                                                                                                                            ]                                        T                                                                              2                                      ⁢                                                      [                  Eqn          .                                          ⁢          2                ]            
In Equation (2), rn denotes an nth sample Rx signal vector in the time domain, Sn−dir,t denotes a symbol transmitted through a Tx antenna ‘t’ prior to dir,t samples, Pr,t denotes the number of resolvable multiple paths between the Tx antenna ‘t’ and an Rx antenna ‘r’, hir,t denotes a channel coefficient of an ith path, Ts denotes a sample period, and dir,t denotes a sample unit delay of the ith path.
When the ML-based MIMO reception is achieved in the time domain, a delay profile of a multi-path channel may vary depending on a pair of Tx/Rx antennas. Eventually, a 2-dimensional space-time domain equalizer is necessary to compensate for the multi-path channel. Thus, a size of a candidate symbol vector of a current sample decreases to |C|NT. However, since candidate symbol vectors are required for up to
      N    s    =            ∑              r        =        1                    N        s              ⁢                  ∑                  t          =          1                          N          r                    ⁢              P                  r          ,          t                    samples detected previously according to the delay profile of the multi-path channel when the ML criterion is determined, there is a shortcoming in that complexity still increases by (|C|NT)NS. To compensate for such a shortcoming, the ML criterion is determined by limiting the candidate symbol vectors only for W samples existing near the current sample without having to consider all symbols in association with the multi-path delay profile.
In FIG. 1B, the linear MIMO reception scheme is still used to separate signals of multiple users, and the ML reception scheme is additionally considered for the purpose of removing Inter-Symbol Interference (ISI) in the time domain. In this case, a MIMO reception performance may still deteriorate since influence on all multiple paths are not considered. A size of a candidate symbol vector required to determine an ML function is (|C|NT)2W+1. Thus, complexity increases exponentially with 2W+1 with respect to |C|NT.
As described above, when the ML-based MIMO detection is performed in the SC-FDMA system, a candidate Tx symbol vector increases exponentially with a length of IDFT in the frequency domain and a computational amount of an ML criterion increases exponentially with a total sum of multiple paths in the time domain, resulting in difficult implementation.